Tetrafluoroethylene polymers are useful when molded into parts for bearings and other mechanical devices where low coefficients of friction and good high temperature stability are desirable. These polymers sometimes contain glass fibers to improve their mechanical properties, such as wear resistance.
These molding powders are ordinarily fabricated in a mold under pressure at room temperatue, followed by sintering the preform at a temperature above the melting point of the polymer, e.g., at about 370-400.degree.C. The sintering can be carried out either while the preform is in the mold, or after the preform has been removed from the mold. In either event, if air is excluded from contact with any portion of the preform, then that portion usually appears discolored, e.g., gray in color, and will contain dark specks when the filler is glass. It is believed that the color and the specks are caused by iron impurity in the glass filler. It is believed that this impurity promotes the decompositon of some of the polymer to elemental carbon, thus resulting in the discoloration and spotting. On the other hand, if air is not excluded from contact with the preform, no discoloration and dark spots are seen on its surface, and it is believed that the oxygen in the air aids in burning off the elemental carbon.
Thus, if the preform is removed from the mold prior to sintering so that its surface is exposed to the atmosphere during sintering, no discoloration or dark specks appear on the surface. However, if the preform is thick enough that the air cannot penetrate into the interior of the preform, then during sintering the air-starved interior will become gray and contain dark specks and upon subsequent cutting, the exposed interior reveals the unsightly specks.
Similarly, if the preform is not removed from the mold prior to sintering, but rather remains in the mold during sintering, then air cannot reach the surface of the preform and upon removal of the sintered preform from the mold, the discoloration and specks are present on the surface of the molded article as well as in the interior.
Means for removing the metallic iron impurities are known, e.g., removal from the glass fiber fillers can be accomplished by cleaning the glass before preparing the filled tetrafluoroethylene resin. However, this introduces a separate step into the process, and furthermore, only removes the impurities from one of several possible sources of contamination.
Frequently the molding powder is agglomerated prior to molding it, to obtain better flowability into the mold.
The purpose of the agglomeration procedure is to place the resin in a form in which it can flow easily through automatic feeding devices into performing machines.
A means for removing the iron impurities and preventing the dark spots and discoloration during the agglomeration step, whatever the source and without any additional process steps, is desirable.